Recently, there is an increased demand for interim storage of spent fuel (hereinafter, referred to as “SF”) in a nuclear power plant. Furthermore, in a recent trend, the interim storage of SF is shifted from wet storage (storage in water) to dry storage (storage with air cooling). Consequently, SF shows a higher calorific value and higher neutron formation density than in the past. Hence, a boron-containing aluminum plate material for forming a cask or a canister as a SF storage container is also required to have higher boron content than in the past.
A melting-and-casting process has been used for manufacturing boron-containing aluminum alloy. The melting-and-casting process includes a process in which powdery boron is mixed in aluminum alloy metal that is then melted and casted (hereinafter, referred to “former melting-and-casting process”), and a process in which a boron fluoride such as KBF4 and a catalyst are mixed into molten aluminum to produce an aluminum-boron intermediate alloy that is then casted while boron concentration is adjusted (hereinafter, referred to “latter melting-and-casting process”). The ingot casted in this way is formed into a plate material through rolling or extruding.
In the former melting-and-casting process, various boron compounds are formed in the aluminum-boron alloy through crystallization and precipitation, leading to degradation in workability. Furthermore, the formed various boron compounds each settle out or surface depending on their specific gravities different from one another, resulting in nonuniform boron distribution (i.e., segregation). As a result, there occurs a portion having a low boron concentration with respect to the amount of added boron, so that actually achievable boron concentration has an upper limit of about 1 mass %.
The latter melting-and-casting process inevitably requires boron (enriched boron) having an increased concentration of boron isotope with a mass number of 10 (hereinafter, referred to “B-10”) which has thermal neutron absorbing power. Such enriched boron, however, is extremely expensive, leading to a cost problem.
Furthermore, the following techniques have been proposed.
There is disclosed a technique for manufacturing an aluminum alloy material, in which aluminum alloy powder containing 0.5 mass % to 5 mass % of boron is produced, a compact is formed of the aluminum alloy powder, and the compact is melted and casted into the aluminum alloy material (see PTL 1). Use of this technique definitely leads to uniform distribution of boron since the powder includes small particles.
In addition, there is disclosed an aluminum-based composite material including a ceramic frame containing a matrix of aluminum or aluminum alloy and a neutron absorbing material such as a boron compound, and a technique for manufacturing the aluminum-based composite material (see PTL 2). The ceramic frame disclosed in PTL 2 is configured as a porous preform produced in such a manner that a slurry is prepared by mixing whisker or short fiber of aluminum borate as ceramics, boron compound particles, and the like, the slurry is dehydrated and pressurized, and the pressurized slurry is sintered into the porous preform. The aluminum-based composite material is manufactured by highly impregnating the ceramic frame formed as the porous preform with molten aluminum or molten aluminum alloy, and casting and solidifying such molten metal into a matrix form.